HCV is a single stranded, positive-sense RNA virus belonging to the Flaviviridae family of viruses in the hepacivirus genus. The NS5B region of the RNA polygene encodes an RNA dependent RNA polymerase (RdRp), which is essential to viral replication. Following the initial acute infection, a majority of infected individuals develop chronic hepatitis because HCV replicates preferentially in hepatocytes but is not directly cytopathic. In particular, the lack of a vigorous T-lymphocyte response and the high propensity of the virus to mutate appear to promote a high rate of chronic infection. Chronic hepatitis can progress to liver fibrosis, leading to cirrhosis, end-stage liver disease and HCC (hepatocellular carcinoma), making it the leading cause of liver transplantations.
There are six major HCV genotypes and more than 50 subtypes, which are differently distributed geographically. HCV genotype 1 is the predominant genotype in Europe and in the US. The extensive genetic heterogeneity of HCV has important diagnostic and clinical implications, perhaps explaining difficulties in vaccine development and the lack of response to current therapy.
Transmission of HCV can occur through contact with contaminated blood or blood products, for example following blood transfusion or intravenous drug use. The introduction of diagnostic tests used in blood screening has led to a downward trend in post-transfusion HCV incidence. However, given the slow progression to the end-stage liver disease, the existing infections will continue to present a serious medical and economic burden for decades.
The first generation HCV therapies were based on (pegylated) interferon-alpha (IFN-α) in combination with ribavirin. This combination therapy yields a sustained virologic response in more than 40% of patients infected by genotype 1 viruses and about 80% of those infected by genotypes 2 and 3. Beside the limited efficacy on HCV genotype 1, this combination therapy has significant side effects and is poorly tolerated in many patients. Major side effects include influenza-like symptoms, hematologic abnormalities and neuropsychiatric symptoms. The second generation of HCV treatments added the HCV protease inhibitors telepravir or boceprevir, allowing treatment times to be shortened, but generating a significant number of serious side-effects. A major improvement in treatment was possible with the introduction of the protease inhibitor simeprevir and the HCV polymerase inhibitor sofosbuvir. These were initially co-administered with interferon and ribavirin, but more recently the co-administration of simeprevir (WO2007/014926) and sofosbuvir (WO2008/121634) has allowed interferon-free and ribavirin-free HCV treatment with further diminished treatment times and dramatically decreased side effects.
An advantage of nucleoside/nucleotide HCV polymerase inhibitors such as sofosbuvir, is that they tend to be active against several of the HCV genotypes. Sofosbuvir for example has been approved by the FDA and EMA for treatment of HCV genotypes 1 and 4. However, in the Fission phase III clinical trials reported in Lawitz et al, N. Eng. J. Med. 2013; 368:1878-87, it was noted “Response rates in the sofosbuvir-ribavirin group were lower among patients with genotype 3 infection than amongst those with genotype 2 infection (56% vs. 97%)”. Hence there is a need for more effective, convenient and better-tolerated treatments.
Experience with HIV drugs, in particular with HIV protease inhibitors, has taught that sub-optimal pharmacokinetics and complex dosing regimes quickly result in inadvertent compliance failures. This in turn means that the 24 hour trough concentration (minimum plasma concentration) for the respective drugs in an HIV regime frequently falls below the IC90 or ED90 threshold for large parts of the day. It is considered that a 24 hour trough level of at least the IC50, and more realistically, the IC90 or ED90, is essential to slow down the development of drug escape mutants. Achieving the necessary pharmacokinetics and drug metabolism to allow such trough levels provides a stringent challenge to drug design.
The NS5B RdRp is absolutely essential for replication of the single-stranded, positive sense HCV RNA genome which makes it an attractive target for the development of antiviral compounds. There are two major classes of NS5B inhibitors: non-nucleoside inhibitors (NNIs) and nucleoside analogues. The NNIs bind to allosteric regions of the protein whereas the nucleoside inhibitors are anabolized to the corresponding nucleotide and act as alternative substrate for the polymerase. The formed nucleotide is then incorporated in the nascent RNA polymer chain and can terminate the growth of the polymer chain. To date, both nucleoside and non-nucleoside inhibitors of NS5B are known.
As stated above, the inhibition mechanism of nucleoside inhibitors involves phosphorylation of the nucleoside to the corresponding triphosphate. The phosphorylation is commonly mediated by host cell kinases and is an absolute requirement for the nucleoside to be active as an alternative substrate for the NS5B polymerase. Typically, the first phosphorylation step, i.e. conversion of the nucleoside to the nucleoside 5′-monophosphate is the rate limiting step. Subsequent conversion of the monophosphate to the di- and tri-phosphate usually proceed facile and are usually not rate limiting. A strategy for increasing nucleoside triphosphate production is to use cell permeable nucleoside prodrugs of the monophosphate, i.e. a nucleoside carrying a masked phosphate moiety, a “prodrug moiety”, which are susceptible to intracellular enzymatic activation leading to a nucleoside monophosphate. The thus formed monophosphate is subsequently converted to the active triphosphate by cellular kinases.
Chemical modifications of an active compound to afford a potential prodrug produces an entirely new molecular entity which can exhibit undesirable physical, chemical and biological properties, thus the identification of optimal prodrugs remains an uncertain and challenging task.
There is a need for HCV inhibitors that may overcome the disadvantages of current HCV therapy such as side effects e.g. toxicity, limited efficacy, lack of pan-genotypic coverage, the emerging of resistance, and compliance failures, as well as improve the sustained viral response.
The present invention provides new HCV inhibiting compounds which have useful properties regarding one or more of the following parameters: antiviral efficacy; pan-genotypic coverage; favourable profile of resistance development; lack of toxicity and genotoxicity; favourable pharmacokinetics and pharmacodynamics; and ease of formulation and administration. The skilled person will appreciate that an HCV inhibiting compound of the present invention need not demonstrate an improvement in every respect over all known compounds but may instead provide a balance of properties which in combination mean that the HCV inhibiting compound is a valuable alternative pharmaceutical agent.
Compounds of the invention may also be attractive due to the fact that they lack activity against other viruses, i.e. are selective, in particular against HIV. HIV infected patients often suffer from co-infections such as HCV. Treatment of such patients with an HCV inhibitor that also inhibits HIV may lead to the emergence of resistant HIV strains.